The present invention relates to coupling light onto an optical detector, and more particularly to a fiber-pigtailed assembly that couples light from an optical fiber onto an optical detector having a relatively large area such that the polarization-dependent responsivity and back reflection are very low.
Polarization-dependent responsivity (PDR) is the ratio of the maximum photo-current divided by the minimum photo-current generated in an optical detector for a given flux of photons over all possible states of polarization and correlates to polarization-dependent loss (PDL). Ideally PDR is zero dB. Reflectivity is the flux of photons that travel backward to the optical source divided by the flux of photons impinging upon the optical detector, usually expressed in dB. Ideally the reflectivity is minus infinite dB.
Low back reflection and low PDR tend to be difficult to achieve simultaneously. Usually methods that improve one make the other worse. For some types of test equipment that measure the characteristics of fiber-optic components it is important to have an optical detector with both low reflectivity and low PDR. Typical requirements for back reflection and PDR are −45 dB and 0.03 dB, respectively. Extraordinary requirements are −65 dB and 0.003 dB, respectively.
While these requirements are important for test-and-measurement (T&M) applications, they are not typically necessary for the majority of people who use fiber-pigtailed detectors. Since T&M applications are a small percentage of the overall market for such devices, it is often difficult or impossible to find a company that can or will build such a device for the limited T&M market.
A common method of reducing back reflection is to use various optical coatings that are designed for low reflectance. These coatings are based on interference, however, and so exhibit wavelength dependence. Even at the optimized wavelength, coatings have difficulty achieving the best-desired specifications. For applications involving wide-band detection, i.e., over many hundreds of nanometers of wavelength, the performance of such coatings is a serious limitation.
Another common method of achieving low back reflection is to tilt the surface of the optical detector relative to the input fiber. The resulting reflected light is thus angled such that it fails to couple to guided fiber modes and is quickly lost. Unlike coatings, tilted surfaces are inherently broadband, resulting in very low reflectivity. Tilting the optical surface, however, introduces PDR. This is largely the result of the fact that the power in the reflected light from a tilted surface is a function of the state of polarization. This is often the source of the low PDR/low back reflection trade off.
In T&M applications using photodetectors to measure optical power requirements typically calls for low back reflection to help avoid instabilities that result from feedback into the laser source, while also calling for low PDR. The problem is that the optimum way of achieving low back reflection, as indicated above, is to use tilted surfaces, yet tilted surfaces introduce polarization dependencies. This problem is amplified in pigtailed optical detectors because the optical pigtail acts like a weak Fabry-Perot resonator, with a birefringent medium (the fiber itself) between mirrors (the reflective ends of the fiber pigtail).
In a Fabry-Perot resonator without birefringence between the mirrors the transmitted and reflected fields add coherently to produce the characteristic wavelength-dependent transmission of these resonators. The introduction of a birefringent material between the mirrors causes relative rotation between reflection/transmission terms that ordinarily would add. However rotation of these terms interferes with their ability to coherently mix, resulting in modulation of the transmitted optical power, i.e., the modulation is a function of the birefringence and wavelength. Thus a simple component made of parts that individually have no polarization-dependent loss (PDL) may exhibit significant PDL. When pigtailed to the optical detector, the fiber assembly and detector together constitute a device that may show considerable PDR. Assemblies with normal incidence, where PDL might be expected to be least, actually show more polarization dependence than those with tilted surfaces. This is because the PDL of the weak Fabry-Perot resonator inherent in the fiber pigtail is amplified by the relatively strong reflection at normal incidence.
What is desired is a fiber-pigtail assembly for an optical detector that provides both low PDR and low back reflection sufficient to meet or exceed even extraordinary specifications.